A cost-effective method for the treatment of reject water from sludge dewatering process using supernatant from sludge lime stabilization

•Sludge lime stabilization supernatant was used as a cheap Ca source for P removal.•High content of calcium and alkalinity in the supernatant favors P precipitation.•Phosphorus removal from reject water achieved 90% by dosing 15% supernatant.•Precipitates transformed from hydroxyapatite to tri-calcium phosphate with rising pH.